A “single photon source” is a component or a system capable of generating light pulses containing one and only one photon. A high performance single photon solid source potentially has many important applications, for example in metrology (light flux or energy standard) or in telecommunications, by enabling absolute security of exchanges using quantum cryptography.
Note that in the telecommunications field, the envisaged applications for quantum cryptography mainly concern emission on optical fibres at usual wavelengths (1.3-1.55 μm), but also by telecommunications in free space (Earth—satellite or inter-satellite communications, submarine communications within the blue-green emission window, short distance land communications).
For this application to quantum cryptography, it is essential that pulses output by the source never contain more than one photon to achieve unconditional confidentiality. At the moment, very attenuated laser pulses containing 0.1 photon on average, and which are only very imperfect approximations of single photon pulses, are used for availability reasons.
These sources have been used to validate the compatibility of quantum communication protocols with the existing optical network, but a Poisson distribution of their pulses causes severe limitations for real use; firstly, 90% of pulses do not contain any photons, which severely limits the transmitted data rate; a more severe problem is that security is compromised by the fact that 1% of pulses (about 10% of useful pulses) contain more than one photon.
Therefore, a spy could intercept the entire transmitted sequence, and could analyse and retransmit the pulses containing several photons without errors. The only action taken by this spy would be to reduce the transmitted rate rather than increase the error rate, and it would be impossible to distinguish his action from the presence of optical losses on the line. Therefore it is important to develop a source capable of delivering pulses that never contain more than one photon, with the highest possible probability of containing one photon.
If single atoms in cavities are capable of producing such a source, the sophistication of the experimental system necessary to prepare, store and manipulate these atoms makes it impossible to consider their application on a large scale. Therefore the deployment of quantum cryptography depends on the development of a low cost high performance solid source of single photons.
A single photon source uses a single emitter (atom, ion, molecule, etc.) to guarantee that the system only stores one elementary electrical or optical excitation, and becomes de-excited later by emitting not more than one photon.
The emission of this localized single emitter is naturally omnidirectional; therefore it needs to be placed in an optical microcavity in order to efficiently collect the emitted radiation, and for example to be able to inject it into an optical fibre. The ideal situation is a situation in which the emission is concentrated in a single mode of the cavity, such that all single photons can be prepared in the same quantum state.
In the past, this combination of a single emitter and a monomode microcavity has not been made successfully, since there are a number of problems:                the atoms, ions or molecules are very imperfect emitters. Molecules have a short life, limited by optical tarnishing or “photodarkening”; the radiation efficiency of atoms and ions placed in a solid matrix is frequently low, and it is difficult to control their numbers;        at the moment, there is no genuinely monomode monolithic optical microcavity. In 1987, Yablonovitch proposed to start from an artificial crystal with a prohibited photonic band (that does not support any electromagnetic mode within a given frequency range), and introduce a single electromagnetic mode in the prohibited band in a controlled manner, by introducing an appropriate defect in the crystal. Although genuine progress had been made in 98-99 in the domain of making crystals with a prohibited three-dimensional photonic band, this approach is found to be technologically very difficult to implement.        
Several approaches have been proposed to solve one of these problems.
It is tempting to replace the usual atom, ion or molecule emitter by a semiconducting emitter, a quantum well or a quantum box that has a radiation efficiency of close to one, and which might be electrically pumpable.
Yamamoto et al. has thus proposed to use a quantum well as the active medium (1), since the electronic states of a well are not discrete, it is then necessary to inject a single electron-hole pair at a time if it is required to obtain a single photon. This result may be obtained using Coulomb blockage to inject exactly one electron and one hole in the quantum well. However, in this approach it appears very difficult to increase the operating temperature above 0.1 K, which strongly reduces its usefulness. J. M. Gérard and B. Gayral also proposed to use a single semiconducting quantum box as an emitting centre ([2]). The quantum boxes obtained by auto-organized growth have many advantages in this context, when they are compared with the most frequently studied atoms, ions or molecules; these advantages include good stability, radiation efficiency very close to one while the heat emission of carriers is negligible (in other words T<150K for the most frequently used InAs quantum boxes emitting at close to 1 μm) and the possibility of non resonant electrical or optical pumping due to the efficient capture of carriers injected in the barrier.
As for the quantum well, there is apparently nothing to prevent the injection of several electron-hole pairs into the quantum box at a time, and observing the emission of several photons.
It is observed experimentally that these photons are emitted at different wavelengths corresponding to different charge states of the quantum box, due to the strong Coulomb interaction between trapped carriers. Therefore, it is sufficient to spectrally filter the emission from a quantum box to observe the emission of a single photon in a well chosen spectral window, after impulse pumping.
J. M. Gérard and B. Gayral also proposed to use the “Purcell effect” (exaltation of the spontaneous emission rate of an emitter in a cavity), to very preferentially collect photons useful in a given mode.
Note that the usual monolithic microcavities (micro-column, micro-disk, micro-sphere, etc.) support a discrete set of confined optical modes and a continuum of leakage modes.
When a monochromatic emitter is put in resonance with a confined mode of the microcavity, a very strong exaltation of the spontaneous emission of the emitter towards this mode is observed under some conditions. In the case of InAs quantum boxes, the inventors observed an emission towards this single confined mode 17 times faster than towards all leakage modes from the microcavity ([3]). This effect makes it possible to couple of the order of 95% (=17/(17+1)) of photons towards the resonant mode of the microcavity.
The inventors thus recently proposed to make a single photon source by putting a single quantum box (for example made of InGaAs) inside an optical microcavity (for example an GaAs/AlAs micro-column) and resonance with a confined mode of this microcavity (the fundamental mode in this case) as shown diagrammatically in FIG. 1 which represents an GaAs/AlAs micro-column, in other words a set containing a microcavity “λ” (in other words a microcavity with a thickness equal to one optical wavelength) made of GaAs, sandwiched between two Bragg mirrors, each composed of an alternating stack of quarter wave layers of GaAs and AlAs. An InGaAs quantum box is placed in this cavity, in resonance with the cavity mode. This micro-column is placed on a GaAs substrate. The diameter of this micro-column is 1 μm.
This association of a quantum box/microcavity can simultaneously achieve the emission of single photons and highly preferential coupling with a given mode, according to the operating method described herein.
Impulse optical pumping of the GaAs barrier is achieved; the photogenerated carriers are very quickly captured by the quantum box (or by the free surfaces of the column). The power of the pump is adjusted so as to inject on average of the order of five electron-hole pairs per pulse into the quantum box, such that the probability of having at least one electron-hole pair in the quantum box is very close to 1.
After recombining the excess pairs, with emission of photons at energies shifted from the cavity mode by the Coulomb interaction between carriers, there is still one pair in the quantum box which is then in resonance with the mode.
Due to the Purcell effect, this final photon is emitted very preferentially in the fundamental mode of the micro-column. Therefore, the system proposed here ideally acts like a converter of Poisson pulses from a pump into a stream of single photon pulses. This converter may be integrated vertically and monolithically with an emission laser through the surface in order to form a micro-source of electrically pumped single photons. An operating temperature equal to at least 77K can be obtained.
It is difficult to isolate a single box in a microcavity. Auto-organized growth techniques usually lead to the manufacture of dense quantum box assemblies (typically 400 boxes per μm2 for InAs/GaAs). The size of these quantum boxes fluctuates; each box has a spectrally very fine emission ray, but their emission wavelength is distributed at random over a wide spectral range (30 to 100 meV in the case of InAs boxes in GaAs). A typical microcavity has a section of a few μm2, and therefore contains of the order of 1 000 quantum boxes; at the centre of the distribution, there will be of the order of 10 quantum boxes in the cavity for a spectral range of 1 meV. Therefore it is necessary to strongly reduce the number of quantum boxes in the cavity if it is important to be sure that there is a single box in resonance with the useful mode. Several solutions are then possible:                a technological approach could be used to reduce the number of boxes (for example the plane of the boxes could be structured before defining the microcavity); all imaginable approaches of this type are very cumbersome, and tend to degrade the quality of the optical microcavity;        in 1999 (ref. [5]), it was proposed to start from a very sparse boxes plane (about 10 boxes per μm2). Spectrally, there is then typically one quantum box every 5 meV, and the probability of observing two quantum boxes with the same emission wavelength is very low. The temperature can then be adjusted (of the order of 20K in practice) so as to modify the emission energy of the quantum box closest to the mode, and bring it into resonance. In one variant, the same effect can be achieved by applying a magnetic field. This approach imposes working within a very narrow range of growth parameters for the quantum boxes plane, in order to obtain a low density. For example, in the case of the InAs/GaAs system, the quantity of InAs deposited must be controlled to within less than 0.03 nm, which may be impossible to achieve. In practice, the quantity of InAs at the surface of epitaxied samples must be varied gradually in order to find a useful area in which the surface density of quantum boxes is sufficiently small. Characterisation and tests of structures become much more difficult, and the efficiency of a manufacturing process based on this approach will be very low.        
It has also been proposed to place one or several quantum boxes in a microcavity to make lasers with a very low threshold current (4). This known structure is very similar to another known structure consisting of surface emission lasers with quantum boxes.